A prior art, U.S. Pat. No. 3,114,311 (Patent Document 1), discloses a technique to obtain a continuous fiber-reinforced resin strand (rod-shaped composition) formed of a twisted resin-impregnated reinforcing fiber bundle by letting a long reinforcing fiber bundle be impregnated with molten thermoplastic resin and pultruding the resulting resin-impregnated reinforcing fiber bundle with the use of twisting rollers while being twisted by the twisting rollers, and to obtain continuous fiber-reinforced thermoplastic resin pellets by cutting the continuous fiber-reinforced resin strand into pellets of a specific length.
More specifically, a long reinforcing fiber bundle is soaked in a molten resin bath filled with thermoplastic resin for letting the fiber bundle be impregnated with molten resin. After the resulting resin-impregnated reinforcing fiber bundle is pulled out from the molten resin bath by way of a forming die, it is rotated in a twisting direction by the twisting rollers rotating in directions inclined with respect to the pulling-out direction, so that the resin-impregnated reinforcing fiber bundle is pultruded with twisting. A continuous fiber-reinforced resin strand (rod-shaped composition (rod)) formed of the twisted resin-impregnated reinforcing fiber bundle is cut into pellets of a specific length (for example, about 3 to 10 mm). Injection molded articles using the continuous fiber-reinforced thermoplastic resin pellets obtained as described above are used, for example, for vehicle interior members (the console box, the instrument panel, etc.), vehicle exterior members (the bumper, the fender, etc.), the housing for electronic device members (a notebook personal computer, a mobile phone, etc.), and so forth.
Because the prior art described above is configured to twist the continuous fiber-reinforced resin strand (rod-shaped composition) by imparting a torque to the strand while the strand is being pultruded, a resin layer is formed on the surface of the continuous fiber-reinforced resin strand. Consequently, it is thought that breaking and fluffing of the reinforced fibers forming the reinforcing fiber bundle occur less and the resin layer exerts a lubrication function between the self and the die, which enables a stable operation at a low pulling-out force (pultruding force).
In the prior art described above, however, depending on the value of a volume filling ratio Vf, which is a proportion of the cross section area of the reinforcing fiber bundle in the cross section area of the pellet cross section in a direction perpendicular to the pellet longitudinal direction, the reinforcing fiber bundle is concentrated in the center portion of the pellet cross section as the result of twisting, which makes the peripheral surface resin layer on the pellet thicker. Hence, when injection molding is performed using such continuous fiber-reinforced thermoplastic resin pellets, the peripheral surface resin layer is so thick that heat transfer to the reinforcing fiber bundle present in the center portion of the pellet is deteriorated when the pellets are heat-melted inside an injection molding machine. This consequently raises a problem that dispersion of the reinforcing fiber bundle in the injection molded articles is poor and the outward appearance and mechanical properties, such as strength, of the injection molded articles are deteriorated.
[Patent Document 1] U.S. Pat. No. 3,114,311 (page 1 and FIG. 1)